This invention generally relates to micromachined ultrasonic transducers. In particular, the invention relates to capacitive micromachined ultrasonic transducers (cMUTs). One specific application for cMUTs is in medical diagnostic ultrasound imaging systems.
Conventional ultrasound imaging transducers generate acoustic energy via a piezoelectric effect in which electrical energy is converted into acoustic energy using a poled piezoelectric ceramic material. The acoustic energy that is transmitted in the forward direction, which is in the direction of the patient being scanned, is coupled to the patient through one or more acoustic matching layers. However, the acoustic energy transmitted in the direction away from the patient being scanned is typically absorbed in and/or scattered in an acoustic backing material located on the backside of the transducer array. This prevents the acoustic energy from being reflected from structures or interfaces behind the transducer and back into the piezoelectric material, thereby reducing the quality of the acoustic image obtained from reflection within the patient. Numerous compositions for the acoustic backing material are known. For example, the acoustic backing material may consist of a composite of metal particles (e.g., tungsten) in an attenuating soft material such as rubber, epoxy or plastic. Other acoustic backing material compositions may also be used.
The ultrasonic transducers used for medical diagnostic imaging have broad bandwidth and high sensitivity to low-level ultrasonic signals, which characteristics enable the production of high-quality images. Piezoelectric materials that satisfy these criteria and have been conventionally used to make ultrasonic transducers include lead zirconate titanate (PZT) ceramic and polyvinylidene fluoride. However, PZT transducers require ceramic manufacturing processes that are different from the processing technologies used to manufacture other parts of an ultrasound system, such as semiconductor components. It is desirable that ultrasonic transducers be manufactured using the same processes used to fabricate the semiconductor components.
Recently semiconductor processes have been used to manufacture ultrasonic transducers of a type known as micromachined ultrasonic transducers (MUTs), which may be of the capacitive (cMUT) or piezoelectric (pMUT) variety. cMUTs are tiny diaphragm-like devices with electrodes that convert the sound vibration of a received ultrasound signal into a modulated capacitance. For transmission the capacitive charge is modulated to vibrate the diaphragm of the device and thereby transmit a sound wave.
One advantage of MUTs is that they can be made using semiconductor fabrication processes, such as microfabrication processes grouped under the heading xe2x80x9cmicromachiningxe2x80x9d. As explained in U.S. Pat. No. 6,359,367:Micromachining is the formation of microscopic structures using a combination or subset of (A) Patterning tools (generally lithography such as projection-aligners or wafer-steppers), and (B) Deposition tools such as PVD (physical vapor deposition), CVD (chemical vapor deposition), LPCVD (low-pressure chemical vapor deposition), PECVD (plasma chemical vapor deposition), and (C) Etching tools such as wet-chemical etching, plasma-etching, ion-milling, sputter-etching or laser-etching. Micromachining is typically performed on substrates or wafers made of silicon, glass, sapphire or ceramic. Such substrates or wafers are generally very flat and smooth and have lateral dimensions in inches. They are usually processed as groups in cassettes as they travel from process tool to process tool. Each substrate can advantageously (but not necessarily) incorporate numerous copies of the product. There are two generic types of micromachining . . . 1) Bulk micromachining wherein the wafer or substrate has large portions of its thickness sculptured, and 2) Surface micromachining wherein the sculpturing is generally limited to the surface, and particularly to thin deposited films on the surface. The micromachining definition used herein includes the use of conventional or known micromachinable materials including silicon, sapphire, glass materials of all types, polymers (such as polyimide), polysilicon, silicon nitride, silicon oxynitride, thin film metals such as aluminum alloys, copper alloys and tungsten, spin-on-glasses (SOGs), implantable or diffused dopants and grown films such as silicon oxides and nitrides.
The same definition of micromachining is adopted herein.
Acoustic energy generated using a capacitive micromachined ultrasonic transducer device does not rely on a piezoelectric material to generate ultrasonic energy. Rather, the basic structure of a cMUT is that of a conductive membrane or diaphragm suspended above a conductive electrode by a small gap. When a voltage is applied between the membrane and the electrode, Coulombic forces attract the membrane to the electrode. If the applied voltage varies in time, so too will the membrane position, generating acoustic energy that radiates from the face of the device as the membrane moves in position. While the acoustic energy is generated primarily in the forward, or patient, direction, some fraction of the acoustic energy will be propagated into the cMUT supporting structure. This structure is commonly a silicon wafer. An acoustic backing material is therefore needed to prevent reflection of this energy from the silicon/air interface at the back surface of the wafer back into the cMUT device. This is equally true for pMUT devices.
U.S. Patent Application Pub. No. US 2002/0048219 discloses a microfabricated acoustic transducer with suppressed substrate modes. This publication discloses the application of acoustic damping material on the backside of the transducer substrate.
A MUT device benefits from a backing material in more ways than simply acoustic performance. A MUT device manufactured on a silicon wafer is fragile and requires great care during the manufacturing process to not become damaged. For example, the electrical connection to the MUT device may be made by lamination of a flexible circuit. During this process, pressure is applied to the silicon wafer to bond electrical connections. Unevenly applied pressure can lead to fracture of the device. The thinned substrate onto which the MUT is built is very fragile and could benefit from a supportive structure.
There is a need for structures and methods for lending additional support to MUT (CMUT and pMUT) devices, while also enhancing the ability to damp undesirable acoustic waves that exit the rear face of the substrate of the MUT device.
The present invention is directed to MUT devices having a body of acoustic backing material and to methods for manufacturing such devices. Various embodiments are disclosed in which an acoustic backing layer or a supporting membrane serve as vehicles for processing very thin substrates without inflicting damage. Other embodiments have acoustic backing material specifically designed for use with MUT arrays on silicon substrates. These features can be combined in one and the same embodiment.
One aspect of the invention is a micromachined ultrasonic transducer device comprising: an ultrasonic transducer array micromachined on a substrate; flexible electrical connections connected to the ultrasonic transducer array; and a body of acoustically attenuative material that supports the substrate and the flexible electrical connections.
Another aspect of the invention is an ultrasound transducer comprising: a silicon substrate; an array of ultrasonic transducer cells supported by the silicon substrate; and a body of acoustically attenuative material disposed on a side of the substrate opposite to the array, the acoustically attenuative material comprising tungsten particles dispersed in a matrix material. The mass percent of tungsten lies in the range of 96.0% to 96.65%, with the remainder being the matrix material. A first fraction of the tungsten particles have a particle size on the order of 1 micron and a second fraction of the tungsten particles have a particle size on the order of 10 microns. Other fillers besides tungsten are also viable as well as other particle sizes.
Yet another aspect of the invention is an ultrasound transducer comprising: a silicon substrate; an array of ultrasonic transducer cells supported by the silicon substrate; a body of acoustically attenuative material disposed on a side of the substrate opposite to the array; and a layer of acoustic impedance matching material disposed between the substrate and the body of acoustically attenuative material. It is preferred that the acoustic impedance of the acoustic impedance matching material has a value that lies between the acoustic impedance of silicon and the acoustic impedance of the acoustically attenuative material.
A further aspect of the invention is an ultrasound transducer comprising: a silicon substrate; an array of ultrasonic transducer cells supported by the silicon substrate;
and a body of acoustically attenuative material disposed on a side of the substrate opposite to the array, the acoustically attenuative material comprising tungsten particles and particles of highly thermally conductive filler material dispersed in a matrix material.
Another aspect of the invention is a method of manufacturing an ultrasound transducer comprising the following steps: attaching a substrate to a layer of acoustic backing material; thinning the substrate while it is attached to the layer of acoustic backing material; and micromachining an array of ultrasonic transducer cells on the substrate.
Yet another aspect of the invention is a method of manufacturing an ultrasound transducer comprising the following steps: micromachining an array of ultrasonic transducer cells on a substrate; attaching a front face of the array to a support membrane; thinning the substrate while supported by the support membrane; attaching a formed or conformal layer of backing material to the thinned substrate; and removing the supporting membrane.
Another aspect of the invention is a micromachined ultrasonic transducer device comprising: an ultrasonic transducer array micromachined on a generally rectangular substrate; flexible electrical connections connected to the ultrasonic transducer array; and a body of acoustically attenuative material that supports the substrate and extends beyond opposing sides of the substrate.
A further aspect of the invention is a method of manufacturing an ultrasound transducer comprising the following steps: attaching a substrate having a first area to a layer of acoustic backing material having a second area greater than said first area, the substrate being placed so that respective portions of the layer of acoustic backing material extend beyond opposing side edges of the substrate; thinning the substrate while it is attached to the layer of acoustic backing material; micromachining an array of ultrasonic transducer cells on the substrate; and trimming off the respective portions of said layer of acoustic backing material subsequent to said micromachining step.
Other aspects of the invention are disclosed and claimed below.